Circuit and method of igniting a high-pressure lamp

ABSTRACT

The invention relates to an electronic circuit for igniting a high-pressure lamp. A resonant circuit  13  of the circuit is used to supply the ignition voltage for the high-pressure lamp. To enable an exact adjustment of the resonance frequency in the resonant circuit, it is proposed that the circuit also comprises converter  12  for generating an alternating voltage with which the resonant circuit  13  is excited and an oscillator  14, 16, 17  for driving the converter  12 , the fundamental frequency of the output voltage of the oscillator being at least in proximity to an integral fraction of the resonance frequency of the resonant circuit  13 . Finally, it is proposed that the circuit includes a feedback  17  from the resonant circuit  13  to the oscillator  14, 16, 17 , whereby the fundamental frequency of the output voltage of the oscillator is so tuned that the resulting frequency of the output voltage of the oscillator corresponds substantially exactly to the integral fraction of the resonance frequency. The invention also relates to a corresponding lighting installation and a corresponding method.

The invention relates to an electronic circuit and a method of ignitinga high-pressure lamp, in particular a UHP (ultra high performance) orHID (high intensity discharge) lamp. The invention also relates to alighting installation.

High-pressure lamps are known from prior art and are used, for example,in projectors. For ignition they require a short-time high voltage of upto several kilovolts. The required ignition voltage is thereforemarkedly higher than the normal operating voltage of the lamp and of itselectronic system.

It is also known from existing practice that resonant circuits are usedto ignite HID and UHP lamps. These circuits achieve a sufficiently highincrease in the electronic operating voltage by exciting the resonancefrequency of the resonance circuit. The high increase in voltageattainable with a resonant circuit when excited with a square-wavevoltage characteristic having a frequency corresponding to the resonancefrequency of the resonant circuit is expressed by$\frac{V_{\max}}{V_{DC}} = {1 + {\frac{2}{\pi} \cdot {Q.}}}$In this equation V_(DC) is the operating voltage and Q the qualityfactor of the resonant circuit.

U.S. Pat. No. 6,160,362 describes an electronic circuit for igniting ahigh-pressure lamp in which a resonant circuit is supplied with voltagefor example, via a bridge circuit. The bridge circuit is switched by acontrol circuit, e.g. a microcontroller. It is proposed that thefrequency of the voltage made available by the bridge circuit is firststarted far above the resonance frequency. The voltage across the lampis detected and the frequency is reduced until the voltage required forignition is reached. It is claimed that in this way the circuit isalways operated at most at the voltage required for ignition. With thisapproach, however, it must be ensured that the frequency of the voltagesupplied by the bridge circuit does not fall below the resonancefrequency. Otherwise there is a danger that components could be damagedby excessively high currents. To ensure that the minimum requiredvoltage is reached and that the frequency is not reduced below theresonance frequency, overdimensioning of the circuit is thereforerequired because of component tolerances.

Advantageously, the resonance of the oscillating circuit is not excitedwith a square-wave voltage characteristic having a frequencycorresponding to the resonance frequency, but with a harmonic of such asquare-wave voltage characteristic. In this way higher resonancefrequencies can be utilized even when using electronic modules whichswitch comparatively slowly. A higher resonance frequency results insubstantially smaller currents during ignition, so that smallercomponents can be used.

When harmonics are used, the relationship between the maximum attainablevoltage and the operating voltage is calculated by:$\frac{V_{\max}}{V_{DC}} = {1 + {\frac{2}{k\quad\pi} \cdot {Q.}}}$In this equation k is the ordinal number of the harmonic used. It isclear from the equation that if a harmonic is used to excite theresonant circuit the quality of the oscillating circuit must becorrespondingly higher to achieve the same maximum voltage.

For example, if the operating voltage V_(DC) is 400 V and the desiredmaximum voltage V_(max) is 5 kV, and if the third harmonic is used, therequired quality factor Q equals 54, whereas if an oscillation havingthe resonance frequency itself were used, Q would equal only 16.

In known electronic circuits for igniting high-pressure lamps theharmonic is made available by means of an oscillator with a fixed setfrequency. The output signal of the oscillator then switches aconverter, to the output of which a resonant circuit of suitable qualityis connected.

However, the quality of the resonant circuit also influences theutilizable bandwidth of the resonant circuit. If the resonance frequencyis not exactly matched, the voltage attained is less than the maximumattainable voltage. A relative frequency error of ½Q, that is, of 0.9%in the above example, is sufficient to cause the attained voltage tofall to a value of 71%. Even normal component tolerances producedeviations above this limit.

It is therefore an object of the invention to make possible a frequencycontrol system for the electronic energization of a resonant circuit bymeans of which the excitation frequency autonomously adjusts itself to afraction of the resonance frequency of the resonant circuit. Anelectronic circuit used for this purpose must be robust, must require nooverdimensioning of the components used, and must prevent the frequencyfrom falling significantly below the resonance frequency of the resonantcircuit.

This object is achieved according to the invention by an electroniccircuit for igniting a high-pressure lamp. The electronic circuitcomprises a resonant circuit for providing an ignition voltage for thehigh-pressure lamp and an converter for generating an alternatingvoltage with which the resonant circuit is excited. The alternatingvoltage generated is preferably of square waveform. In addition, theelectronic circuit according to the invention includes an oscillator todrive the converter. The fundamental frequency of the output voltage ofthe oscillator must be at least in proximity to an integral fraction ofthe resonance frequency of the resonant circuit. Finally, the electroniccircuit according to the invention comprises a feedback from theresonant circuit to the oscillator. On the basis of this feedback, thefundamental frequency of the output voltage of the oscillator is tunedin such a way that the resulting frequency of the output voltage of theoscillator corresponds substantially exactly to the above-mentionedintegral fraction of the resonance frequency.

A further object of the invention is achieved by a lighting installationwhich includes, in addition to the proposed electronic circuit, ahigh-pressure lamp connected to the resonant circuit of the electroniccircuit.

This object is likewise achieved according to the invention by acorresponding method of igniting a high-pressure lamp by means of aresonant circuit supplied with an alternating voltage by a converter.

The invention is based on the idea that a feedback from the resonantcircuit to the oscillator can be used to tune the frequency of anoscillation to a fraction of the actual resonance frequency. Althoughthe actual resonance frequency of a resonant circuit cannot be preciselypredetermined because of component tolerances, such a feedback, whichsupplies to the oscillator an indication of the frequency and the phaseposition of the oscillation in the resonant circuit, permits a preciseadjustment of the excitation of the resonant circuit.

The fundamental frequency of the oscillator should already be close toan integral fraction of the resonance frequency. Otherwise, because ofthe feedback, the oscillator could adjust itself to a fraction above orbelow the anticipated fraction of the resonance frequency. The qualityof the resonant circuit, in the case of an optimized circuit, isinsufficient for a frequency at a fraction of the resonance frequency ofa higher order than anticipated so that the required maximum voltage isnot attained. In the case of an optimized circuit, damage to thecomponents of the circuit is to be feared for a frequency at a fractionof the resonance frequency of a lower order than anticipated since inthat case excessive power dissipation occurs.

It is an advantage of the invention that it permits a precise andautonomous adjustment of the harmonic to the resonance frequency. Thismakes possible a robust electronic circuit, since the integral fractionof the resonance frequency can be selected to be tuned to a desiredquality of the resonant circuit and to a selected dimensioning of thecomponents. Alternatively, given a selected integral fraction of theresonance frequency, the quality of the resonant circuit can be limitedto the required minimum and overdimensioning of the components avoided.

Advantageous embodiments of the circuit and of the method according tothe invention are the subject matter of the dependent claims.

The feedback should be able to measure, as far as possible withoutlosses, the high voltages of, for example, up to 5 kV and to reduce themto a level which is suitable for processing by normal small-signalelectronic circuits. At the same time, however, sufficient feedbackshould be ensured even at relatively low voltages, since the resonantcircuit is not initially in resonance and therefore supplies only lowvoltages of, for example, 400 V.

In a preferred embodiment of the invention, the feedback is thereforeeffected by means of a capacitive antenna with which the frequency inthe resonant circuit can be reliably detected even in the case of largevoltage differences. Moreover, a capacitive antenna has low losses andmakes the electronic circuit less sensitive to other capacitiveinfluences which could occur at the high voltages.

If the electronic circuit is a printed circuit, the capacitive antennaadvantageously takes the form of a suitable conductor trackconfiguration. Such a conductor track configuration is cheaper and morereliable than a capacitance element constructed as a separate unit.

Although the circuit according to the invention allows the resonantcircuit to be put reliably into resonance mode, the maximum voltageresulting from resonance cannot be exactly predetermined. The reasonsare, firstly, component tolerances and, secondly, external influencessuch as air humidity. In addition, as the temperature of the resonancecoil of the resonant circuit increases, the maximum voltage initiallyincreases. Once a certain voltage is exceeded, however, this can causedamage to the components of the resonant circuit.

In a preferred embodiment of the electronic circuit according to theinvention, therefore, the magnetic material of the resonance coil is soselected that it reaches saturation precisely when a desired voltage isreached. The desired voltage is normally the voltage required forignition of the high-pressure lamp. In this way it can be ensured that amaximum value for the voltage of, for example, 5 kV is not exceeded. Inaddition, the saturation of the magnetic material has the advantagethat, with rising temperatures, it initially decreases, inversely to thevoltage, leading to a compensation of the two effects.

Through the adjustment of the saturation of the magnetic material to adesired voltage, therefore, further voltage control becomes superfluous.

The oscillator of the electronic circuit according to the invention canbe realized by means of an analog comparator, but also by means of adigital PLL (phase locked loop). If a PLL is used, both the output ofthe inverter and the output of the resonant circuit should be capturedfor the feedback in order to enable on this basis an adjustment to aphase difference of exactly 90°.

These and other aspects of the invention are apparent from and will beelucidated with reference to the embodiments described hereinafter.

In the drawings:

FIG. 1 shows a block diagram of a first embodiment of the circuitaccording to the invention having an analogue comparator;

FIG. 2 shows an embodiment of an oscillator of the circuit according tothe invention;

FIG. 3 shows the voltages occurring at the oscillator from FIG. 2, and

FIG. 4 shows a block diagram of a second embodiment of the circuitaccording to the invention having a PLL controller.

The block diagram in FIG. 1 illustrates schematically a first embodimentof the electronic circuit according to the invention which is to be usedto ignite a high-pressure lamp.

The electronic circuit illustrated includes a direct voltage source 11which is connected to an inverter 12 which is drivable by means ofswitching elements. A resonant circuit 13 which is used to supply theignition voltage for the high-pressure lamp is connected to the outputof the inverter 12.

An antenna 17 which takes the form of the open end of a copper surfaceon the circuit board is used as the detector for the actual resonancefrequency and is connected to a comparator 14 via a matching circuit 16.The output of the comparator 14, which is provided with a hysteresis, isback-coupled to the matching circuit 16. In addition, the output signalof the comparator 14 is fed back to the switching elements of theconverter 12 via a delay unit 15.

The matching circuit 16 and the comparator 14 together form anoscillator which operates without tuning at least in proximity to anintegral fraction of the anticipated resonance frequency of the resonantcircuit 13.

The electronic circuit of FIG. 1 may form part of a lightinginstallation in which a high-pressure lamp (not shown) is connected tothe resonant circuit 13.

For the frequency control system according to the invention, the antenna17 captures the voltage characteristic across the resonant circuit andfeeds it to the matching circuit 16. Although high voltages in theresonant circuit are captured by the antenna 17 with an attenuation ofamplitude because of the capacitive configuration of the antenna 17, thevoltage characteristic which is passed on does reflect the frequency inthe resonant circuit for each voltage. The matching circuit 16 passes onthe voltage characteristic received to the comparator 14 in such a waythat the zero transitions coincide exactly in time with the switching ofthe comparator 14 if the output frequency of the comparator 14corresponds exactly to an integral fraction of the resonance frequencyof the resonant circuit 13.

To achieve this, the phase position of the coupling-in of the antennasignal is so selected that a lag of the zero transition, indicating thatthe resonance frequency is lower than the actual oscillator frequency,causes a delay of switching by the comparator. By contrast, a lead ofthe zero transition, indicating that the resonance frequency is higherthan the actual oscillator frequency, causes an acceleration ofswitching by the comparator. The phase position of the voltage signalcaptured by the antenna 17 is offset by 90° with respect to the signalat the output of the converter 12 if the resulting switching frequencyis exactly an integral fraction of the natural resonance frequency ofthe oscillating circuit.

The output signal of the comparator 14, the switching times of which aretherefore precisely tuned to a fraction of the resonance frequency ofthe resonant circuit 13, is then used to trigger the converter 12 at afraction of the resonance frequency.

The final phase position of the signal with which the converter 12 istriggered is determined by programming of the delay unit 15 of FIG. 1.

As compared to the high voltage of the resonant circuit 13, the outputsignal of the comparator 14 has a phase shift of 90° if the oscillatoris exactly adjusted to an integral fraction of the resonance frequency.If the oscillator frequency is far above an integral fraction of theresonance frequency, the output signal of the comparator 14 isphase-shifted by 180° with respect to the high voltage of the resonantcircuit 13. If, however, the oscillator frequency is far below anintegral fraction of the resonance frequency, the output signal of thecomparator 14 is not phase-shifted with respect to the high voltage ofthe resonant circuit 13.

To match the phase of the output signal of the comparator 14 to thephase required at the converter 12 for exciting the resonant circuit 13with a harmonic of the resonance frequency, the delay unit 15 thereforeprovides a phase shift of 90° or a corresponding phase shift of90°+n*180°, where n is a natural number. However, phase shifts of morethan 270° are less advantageous because of the resulting long delaybetween measurement and triggering.

To achieve that the delay times unavoidably occurring during processingin the circuit do not prevent exact tuning, the delay unit 15additionally causes a corresponding compensation of the delay in thesignal path from the comparator 14 to the converter 12. This can beachieved in that a fixed phase shift of, for example, 270°, reduced bythe previously determined, design-dependent delay times in the circuit,is set in the delay unit 15. The inversion required because of theopposite signs at 90° and at 270° can be effected, for example, in theconverter.

FIG. 2 shows a possible configuration of the oscillator formed by theantenna 17, the matching circuit 16 and the comparator 14. The analogoscillator shown in FIG. 2 is used to excite the resonance in theresonant circuit 13 with a harmonic of the resonance frequency having anordinal number of k=3.

In the oscillator circuit a capacitor C32 of 0.1 pF, which representsthe capacitive antenna 17, is connected via a resistor R32 to a firstinput 2 of an operational amplifier 21 functioning as a comparator. Theconnection between the capacitor C32 and the resistor R32 is connectedto ground via a further capacitor C37 of 120 pF. The two capacitors C32and C37 form a potential divider for the voltage captured by theantenna.

The connection between the resistor R32 and the first input 2 of theoperational amplifier 21 is connected on one side to ground via aresistor R20 and on the other to a first supply voltage via a resistorR19. In addition, a further resistor R1 is connected to the output 7 ofthe operational amplifier 21 via a capacitor C38. The resistors R19, R20and R1 together with the capacitor C38 produce the hysteresis of thecomparator 14. At the same time the capacitor C38 prevents a reaction bydirect voltage components, so that the symmetry of the output voltage ofthe operational amplifier 21 and therefore of the whole oscillator isensured.

A second supply voltage is fed to the operational amplifier 21.

In addition, the output 7 of the operational amplifier 21 is fed back tothe second input 3 of the operational amplifier 21 via a resistor R18,whereby the output frequency of the operational amplifier 21 can betuned to the resonance frequency of the resonant circuit 13 captured viathe capacitive antenna C32. The connection between the resistor R18 andthe second input 3 is connected to ground via a frequency-determiningcapacitor C37 of 100 pF. The voltage across the capacitor C27 is thusapplied to the second input 3 of the operational amplifier 21.

As long as no signal is received via the antenna C32, a square-waveoutput signal of a certain fundamental frequency is produced at theoutput 7 of the operational amplifier 21. Because of the components ofthe capacitor C27 and of the resistor R18 used for the hysteresiscircuit, the fundamental frequency of the output signal is in proximityto ⅓ of the anticipated resonance frequency of the resonant circuit.

In FIG. 3 the voltages occurring at the oscillator during frequencycontrol are represented in a diagram. In the diagram the voltagesconcerned are plotted as four curves 31 to 34 in volts against time inμs.

Curve 31 represents the signal captured by the capacitive antenna C32.This signal 31 is composed of a substantially sinusoidal oscillationwith an amplitude of approx. 8 V and a direct voltage component ofapprox. 2.5 V.

Curve 32 represents the signal at the first input 2 of the operationalamplifier 21. Because of the potential divider, this signal 32 is,firstly, attenuated as compared to the signal 31 captured by the antennaC32; secondly, superposed on it is an additional, alternating directvoltage component which is produced by the feedback of the output 7 ofthe operational amplifier 21 via the hysteresis circuit. The wholedirect voltage component is switched, approximately after threesemioscillations of the high-voltage signal captured by the antenna,from a higher value of approx. 3V to a lower value of approx. 2V andinversely. The value of the additional, alternating direct voltagecomponent is therefore approx. ±0.5 V.

Curve 33 represents the square-wave output signal of the operationalamplifier 21 at output 7, which signal results from the comparison ofthe voltages supplied to the first input 2 and the second input 3 of theoperational amplifier 21. The square-wave output signal 33 alternatesbetween approx. 0 V and approx. 5 V.

Finally, curve 34 represents the voltage fed back from the output 7 ofthe operational amplifier 21, which has a triangular waveform as aresult of the capacitor C27 and is fed to the second input 3 of theoperational amplifier 21. The characteristic of the triangular-waveformvoltage moves in each wave from below 2 V to above 3 V and back.

Curves 32 and 34 intersect in each case at approximately the zerotransition of the voltage 31 captured via the antenna C32, and switchingof the operational amplifier 21 is effected at each intersection. Thefeedback ensures that the switching is shifted exactly to the zerotransitions of the signal 31. As soon as the switching occurs exactlysimultaneously with the zero transitions of the voltage captured by theantenna 7, the output frequency of the comparator is exactly the desiredintegral fraction of the resonance frequency of ⅓.

FIG. 4 shows a second embodiment of the electronic circuit according tothe invention, based on the use of a PLL.

The circuit of FIG. 4 again comprises a direct voltage source 11 whichis connected to a resonant circuit 13 via an converter 12. Likewise, thecircuit again includes an antenna 17 as detector for the actualresonance frequency.

In contrast to the first embodiment, however, the output signal of theantenna 17 is here fed to a PLL controller 18. The output signal of theconverter 12 is also fed to the PLL controller 18. In addition, theoutput signal of the PLL controller 18 is fed back directly to theswitching elements of the converter 12.

The PLL controller 18 is so dimensioned that a frequency, based on adesired fraction of the anticipated resonance frequency, is produced atits output, at which frequency the inverter output signal and thevoltage at the resonant circuit 13 have a phase shift of 90°. This makescompensation of the other delays in the circuit by means of a delay unitunnecessary.

The electronic circuit of FIG. 4 may also form part of a lightinginstallation in which a high-pressure lamp (not shown) is connected tothe resonant circuit 13.

The embodiments described represent only two of various possibleembodiments of the invention.

1. An electronic circuit for igniting a high-pressure lamp, comprisingresonant circuit (13) for providing an ignition voltage for thehigh-pressure lamp; a converter (12) for generating an alternatingvoltage with which the resonant circuit (13) is excited; an oscillator(14, 16, 17) for driving the converter (12), the fundamental frequencyof the output voltage of the oscillator (14, 16, 17) lying at least inproximity to an integral fraction of the resonance frequency of theresonant circuit (13), and a feedback (17) from the resonant circuit(13) to the oscillator (14, 16, 17), whereby the fundamental frequencyof the output voltage of the oscillator (14, 16, 17) is so tuned thatthe resulting frequency of the output voltage of the oscillator (14, 16,17) corresponds substantially exactly to the integral fraction of theresonance frequency in proximity to which the fundamental frequency ofthe output voltage of the oscillator (14, 16, 17) lies.
 2. An electroniccircuit as claimed in claim 1, characterized in that the feedbackcoupling comprises a capacitive antenna (17) for detecting the outputvoltage of the resonant circuit (13).
 3. An electronic circuit asclaimed in claim 2, characterized in that the capacitive antenna (17) isformed by a conductor track configuration on a printed circuit.
 4. Anelectronic circuit as claimed in any claim 1, characterized in that adelay unit (15) is contained in the signal path from the oscillator (14,16, 17) to the converter (12).
 5. An electronic circuit as claimed inclaim 1, characterized in that the oscillator (14, 16, 17) comprises ananalog comparator (14, 21).
 6. An electronic circuit as claimed in claim5, characterized in that the oscillator (14, 16, 17) has a hysteresiswhich determines the integral fraction of the resonance frequency inproximity to which the fundamental frequency of the output voltage ofthe oscillator (14, 16, 17) lies.
 7. An electronic circuit as claimed inclaim 1, characterized in that the oscillator comprises a digitalcircuit, in particular a digital PLL (phase locked loop) (18).
 8. Anelectronic circuit as claimed in claim 7, characterized in that thedigital circuit (18) utilizes both the signal of the resonant circuit(13) and the output signal of the converter (12) for frequency control.9. An electronic circuit as claimed in claim 1, characterized in thatthe resonant circuit (13) comprises a resonance coil whose the magneticmaterial reaches saturation at a predefined voltage, in particular theignition voltage determined for the high-pressure lamp.
 10. A lightingsystem comprising an electronic circuit as claimed in claim 1 andcomprising a high-pressure lamp connected to the resonant circuit (13)of the electronic circuit.
 11. A method of igniting a high-pressure lampby means of a resonant circuit (13) supplied with an alternating voltageby an converter (12), the method comprising the following steps: a)driving of the converter (12) by a voltage which is output by anoscillator (14, 16, 17) such that the converter (12) outputs analternating voltage, the fundamental frequency of the output voltage ofthe oscillator (14, 16, 17) lying at least in proximity to a fraction ofthe resonance frequency of the resonant circuit (13); b) supplying ofthe alternating voltage output by the converter (12) to the resonantcircuit (13) so as to excite an oscillation in the resonant circuit(13); c) feeding the oscillation caused in the resonant circuit (13)back to the oscillator (14, 16, 17), such that the frequency of theoutput voltage of the oscillator (14, 16, 17) is adjusted to a valuewhich corresponds substantially exactly to the fraction of the resonancefrequency of the resonant circuit in proximity to which the fundamentalfrequency of the output voltage of the oscillator (14, 16, 17) lies, andd) repetition of steps a) to c) until a voltage required for ignition ofthe high-pressure lamp is attained at the resonant circuit (13).
 12. Amethod as claimed in claim 11, characterized in that in step a) thesignal output by the oscillator (14, 16, 17) is transmitted with a delayfor driving the converter (12).